Tire Size Comparison, Speedometer Error and Gearing

Why the drawing, and not just a table

Two tire sizes differing by half an inch look identical in a table of numbers and obviously different when you draw them at the same scale on the same ground line. That is the whole reason the comparison above leads with a picture: a percentage is abstract, and a circle that visibly stands taller than the one next to it is not.

The drawing carries three things at once. The outer circles are the tires at their computed overall diameters. The inner circles are the wheels, which is where most of the surprise lives — put a 20-inch wheel next to a 17 and the sidewall you have left is startlingly thin. The bars underneath are section width at the same scale, because width is invisible in a side view and is usually the dimension somebody forgets is changing.

Neither tire is drawn in a "good" colour. One is a solid line and one is dashed, and that is the only difference between them. This site does not tell you which of two tires is the right answer.

What the numbers are, precisely

Overall diameter is computed from the size code, not looked up. For a metric size such as 285/70R17, the sidewall is 70% of 285 mm, that height appears twice — once above the wheel and once below — and the wheel diameter is added in the middle. For an imperial size such as 33x12.50R17, the diameter is simply the first number, because that is what the notation states.

Revolutions per mile is 63,360 divided by the circumference in inches. It is a geometric figure. A real tire's published revolutions-per-mile is measured on a rolling road and typically runs a few percent higher, because a loaded tire deflects and its rolling radius is smaller than its free radius. Where a manufacturer publishes a measured figure for a specific tire, that figure is better than this one.

Speedometer error is the ratio of the two diameters and nothing else. A speedometer counts wheel revolutions, so if the new tire covers more ground per revolution, the instrument reads low by exactly the proportion the tire grew. Whether the reading is corrected by a dealer tool, a tuner, or not at all is a separate question this site does not answer.

The one thing this site will not do

It will not tell you whether a tire fits. Clearance depends on the specific vehicle, its suspension travel, its steering lock, the wheel's offset and backspacing, and whether anything has been changed already. Load rating depends on the vehicle's own placard. Road-legality depends on where you live. None of those are geometry, and none of them can be derived from two size codes.

What the arithmetic can do is tell you exactly how much bigger the tire is, exactly how far the speedometer will be out, and exactly what ratio would restore the engine speed you had. Those are the questions with a single correct answer, and they are the ones worth computing carefully.

Where to go next

The comparison tool above is the fastest route to an answer for any pairing, including the thousands this site has no page for. The pages that do exist are the ones people search for by name: a page per commonly-sold size, a page per pairing that comes up again and again on a popular platform, and a short answer page for the single-number questions.

If you want to understand the sidewall rather than just measure it, start with the reference section. If you want the arithmetic worked through with a real example, the writing covers the questions in order of how often they are asked.

Common questions

Is 285/70R17 a 33? Almost. It measures 32.71 in against a genuine 33x12.50R17's 33.00 in. The height gap is 0.29 in and the width gap is 1.28 in, which is the part people do not expect.

Two of us measured the same size and got different heights. Who is wrong? Possibly neither. A size code specifies a casing, not a finished tire: tread pattern, brand, remaining tread depth and the width of the wheel it is stretched over all move a tape-measure reading by a few tenths. Every figure on this site is the code's figure, which is exact about the specification and approximate about the object.

Do I need to re-gear? That is a judgement, not a calculation. What is calculable is the ratio that would restore your original engine speed, and whether a ratio close to it is actually sold. The gear calculator gives you both.

Is the load index part of the size? No, and this catches people out. The 285/70R17 part is geometry; the figures that follow it — something in the shape of 121/118S — are the service description, and two tires wearing the identical size code can be rated for very different loads at very different pressures. The load and speed index decoder reads that second half.